The invention relates to a vacuum device comprising a plurality of cryopumps for producing a vacuum.
Such vacuum devices comprise a plurality of cryopumps normally arranged in parallel to each other, said cryopumps being connected with one or a plurality of vacuum chambers. Further, the vacuum device comprises a compressor means with the aid of which the cooling media, normally helium, is compressed. The compressed cooling media is fed via media supply conduits to the cryopumps, expands in the cryopump, and is then returned via media return conduits to the compressor means. Cleaning means may be provided in the media conduit for removing e.g. oil or other contaminants from the media. In this manner, contaminants contained in the media are prevented from entering the cryopumps.
Normally, the cryopumps employed are two-stage cryopumps which operate according to the Gifford McMahon principle. In the cryopump one piston, a shared piston where appropriate, is normally provided for each stage. During each piston stroke a cooling media is transported, and the two stages are cooled correspondingly. For example, radiation heat or other temperature influences may heat up individual pumps. Further, there exists the problem that due to the higher density of a lower-temperature gas, a colder cryopump is capable of processing a larger amount of helium per stroke than a warmer cryopump. Consequently, the available amount of helium, which is limited by the compressor capacity, is consumed to a larger extent by the colder cryopumps such that the amount of gas available for the warmer cryopumps is reduced. As a result, cooling of cryopumps, which are too warm, takes a relatively long time.
According to U.S. Pat. No. 5,775,109 this problem is solved by controlling the gas flow. This control can be effected by heating up the cryopump to prevent the pump from delivering an increased amount of cooling media. Further, the velocity of the piston can be reduced, or the piston can be stopped. This however has the drawback that the thermodynamic efficiency decreases since the coolers are adjusted to a specific frequency. The cooling energy stored in the helium is thus not completely utilized.
It is an object of the invention to provide a vacuum device comprising a plurality of cryopumps, wherein the temperature of the cryopump can be controlled in an easy and rapid manner.